A DC-AC converter is commonly referred to as an inverter. One type of inverter is known as a single phase H-bridge inverter, comprising two DC terminals Tdc1, Tdc2, two AC terminals Tac1, Tac2 and four switches S1, S2, S3 and S4 in the configuration shown in FIG. 1. In most configurations, the switches are transistors with intrinsic diodes, alternatively diodes separate from the switches may be used. In FIG. 1, both the switches and the diodes are illustrated. A corresponding three phase H-bridge inverter is also known.
One common way of controlling the switches of the inverter in FIG. 1 is to use pulse-width modulation (PWM) switching. Here, a first pair of opposite switches (S1 and S4) is controlled by PWM switching and a second pair of opposite switches (S2 and S3) is turned off for one switching period and vice versa for the next switching period.
The inverter can be connected to an AC grid, typically when the inverter is used to convert DC power from solar power systems and then supplied to the AC grid. Here, the AC frequency and voltage amplitude is determined by the AC grid and the inverter is controlling the output current.
The inverter can also be connected to an AC load, typically when the inverter is used in an uninterruptable power supply (UPS) for converting DC power from a battery to a load requiring an AC voltage. Here, the current is determined by the load and the inverter is controlling the output voltage.
It is an object of the invention to achieve an inverter where it is possible to adjust the cosine phi of the AC terminals, while still achieve a high efficiency. Moreover, it is an object to adjust the flow of reactive power between the inverter and the AC side in both directions, i.e. the inverter may either consume or supply the reactive power.